High voltage transients, often called voltage surges, may occur on transmission lines due to lightning strikes, power line switching or other unanticipated events. These voltage surges may cause damage to expensive communication equipment connected to the transmission lines, such as telephones, facsimile machines, or modems.
Overvoltage protection devices have been developed to protect expensive communication equipment from these voltage surges. Such overvoltage protection devices will protect this equipment by diverting the voltage surges away from the transmission line before it has an opportunity to damage the communication equipment. Typically, overvoltage protectors remain in a substantially non-conducting state until a voltage surge occurs on the transmission line. When a voltage surge occurs on the transmission line, the overvoltage protectors will activate into a conducting state thereby diverting the voltage surge away from the transmission line and the telephone equipment.
Conventional overvoltage protection devices include zener diodes, gas discharge tubes and metal oxide varistors ("MOVs"). Zener diodes operate using a voltage breakover point, but they are not suitable for transient suppression because of DC power dissipation and relatively low thermal capacitance. MOVs and gas discharge tubes may not have adequate response capabilities to provide effective protection for fast rising voltage surges. MOVs degrade with operation and gas discharge tubes degrade over time. Such degradations ultimately lead to a loss of protection capabilities.
The thyristor family of semiconductor devices solves many of the problems associated with conventional overvoltage devices. The thyristor family of semiconductor devices consists of several very useful devices, including silicon control rectifiers ("SCRs"), triacs and sidacs. Thyristors are characterized as having two states, an ON state and an OFF state. In the OFF state, the device has high impedance and low current characteristics. In the ON state, the semiconductor device has low resistance and high current characteristics. Some thyristors activate into an ON state by the application of a gating current on the device. Other thyristors automatically activate into an ON state without gating if a large overvoltage surge is placed across the terminals of the device. The thyristor, like conventional overvoltage protectors, is normally in an OFF state allowing only a small leakage current to pass through the device. The thyristor is connected to transmission lines in such a manner as to activate into an ON state when a large overvoltage surge is detected thereby conducting the voltage surge away from communication equipment.
An SCR device is a unidirectional semiconductor device having an anode, a cathode and a gate. In the OFF state, an SCR is a high resistance, low current circuit element. If a momentary positive pulse is applied to the gate of an SCR under the proper voltage conditions, the device will switch to the ON state and become a low resistance, high current element. Once an SCR is activated into the ON state, it will remain in the ON state until the principal anode to cathode current drops below a holding current of the device.
A triac is a bidirectional thyristor which controls power in an AC electric circuit. The operation of a triac can be related to two SCRs connected in an inverse parallel circuit configuration. A triac has a single gate, and the device can be triggered to the ON state by a gate pulse of either positive or negative polarity. In normal operation when a voltage is applied across the top and bottom contacts of the triac, the semiconductor blocks current flow, except for a small leakage current. When a gate current is applied to the single gate of the triac under the proper voltage conditions, the device is triggered into the ON state. The triac will remain in the ON state, even after the gate voltage is removed, until the current drops below a sustaining current, called a holding current (I.sub.H). A triac is bidirectional thereby allowing current flow in either direction.
A sidac has the same low voltage, low current triggering characteristics and high holding characteristics as a triac. The sidac is designed to provide fabrication and installation cost savings by combining several circuit elements into a single two terminal device which functions effectively in an overvoltage protection system. The sidac also eliminates the need for a third contact or gate. Switching of the sidac occurs in the absence of a gate signal when a large voltage potential, in excess of the breakover voltage, is applied across the terminals of the device. This switching phenomenon is commonly known as breakover and the potential at which it normally occurs is known as the breakover voltage.
Semiconductor thyristor devices are characterized generally as having four layers of alternating conductivity, i.e., NPNP or PNPN. The four layers in the sidac device include an emitter layer, an upper base layer, a mid-region layer and a lower base layer, respectively. The emitter region is sometimes referred to as a cathode region, while the lower base layer is sometimes referred to as an anode region. The two metal terminals of the sidac device are coupled to the emitter and lower base regions of the device, respectively.
As the voltage across the device increases, the electric field across the center junction of the device (the upper base layer/mid region layer), reaches a value sufficient to cause avalanche multiplication. After avalanche multiplication occurs, the impedance of the device decreases substantially and the voltage across the device reduces from a maximum voltage value (which is determined by the avalanche voltage of the center junction) to a much lower voltage value. Of course, the current flow through the device will increase proportionally as the voltage across the terminal decreases. The device will remain in this low impedance condition until the current across the device is reduced below a holding current value.
The device switches from an OFF to an ON state when its current gain exceeds unity. The well known method of switching the device from an OFF to an ON state is to increase the voltage across the device terminals until the center junction thereof breaks down or avalanches. In the avalanche condition, the current increases rapidly thereby exceeding the unity value for current gain.
Another method of switching the device from an OFF to an ON state is to apply a signal voltage which is relatively steep, namely, which has a "rate of rise" greater than a predetermined value. "Rate of rise" is often calculated using the function dV/dt, a derivative function of change in voltage over change in time. At equilibrium, the junctions in the device have a space charge region which acts as a built-in capacitor. The charge on this capacitor is related to an applied voltage as follows: EQU Q=C V
As the voltage between the terminals of the device increases, the time rate of change of charge, or current density, also increases. Assuming the capacitance remains constant, the equation for current density, J, is as follows: EQU J=C dV/dt
In this equation, C represents the junction capacitance, V represents the applied voltage and J represents the current density (time rate of change of the charge Q). For a fast rising voltage, the value of J can be substantial. If the "rate of rise", dV/dt of the applied voltage to the device is sufficient, the alpha or current gain of the device will exceed unity. Thus, the device may switch to an ON state if the "rate of rise" applied to the device is sufficiently large.
It has been found that if the switching voltage is approached slowly, the device will turn ON at only one point of the junction. This point corresponds to the junction's lowest voltage breakover voltage. In this situation, the uniform activation of the device will likely be achieved at, or near, the desired breakover voltage.
If, however, a switching voltage is applied to the device with a fast rising voltage pulse having a high "rate of rise", the device will activate at an overshoot voltage significantly above the breakover voltage. In the overshoot voltage situation, the device will not activate at, or near, the desired voltage. The voltage overshoot of the device may significantly affect the amount of voltage allowed to build up before the device turns ON. Such voltage overshoots should be minimized so that any voltage over the breakover voltage of the device does not reach the communication equipment.
Apart from overshoots caused by a high "rate of rise," other differences between the actual breakover voltage and the desired breakover voltages may be caused by inhomogeneities produced by small localized areas or defects. These localized regions may be caused by statistical variations in the concentration of impurities added to a layer in the semiconductor device. Other defects affecting the breakover voltage may be caused by dislocations; metal precipitates such as iron, copper or manganese; oxygen or silicon oxide precipitates within the device; nonuniform doping which may occur during the diffusion process; surface conditions during device fabrication; or, a combination of these factors.
Typically, when the device initially is switched ON, only part of the avalanching junction will be conducting. The remainder of the junction will be brought into full conduction by a spreading of the conducting plasma across that junction. This spreading of conducting plasma corresponds to the spreading of the voltage drop laterally across the junction of the device. The time for the entire junction to be fully activated to the ON state is called the spreading time and the speed of the plasma spreading is called the spreading velocity.
In thyristors, the spreading time is of considerable interest since it has an important effect on the dynamic behavior of the device. The switching speed of the device is directly proportional to the spreading speed of the conducting plasma. Depending on the size of the device, it may take up to several hundred microseconds for the spreading to be completed. During the spreading phase, the voltage across the device is much greater than it is when the thyristor is fully activated across the junction. The spreading velocity should be maximized to minimize the spreading time. A shorter spreading time should minimize the voltage build up in the device prior to activation. Further, spreading velocities should be maximized to minimize the time necessary to activate the device during very high surge concentrations.
The presence of hot spots and defects reduces the switching speed of the device by producing avalanching at specific locations on the device junction while not avalanching at other locations on the junction. These defects and hot spots hinder the spreading of the conducting plasma. Further, these hot spots and other defects will initially carry most of the current and continue to carry this current until the voltage drop spreads laterally across the remainder of the junction. These defects produce excessive local current densities and ultimately burnout of the device. The device may not fully activate into an ON state due to these localized hot spots; which, in turn may cause the overshoot voltage to increase substantially over the desired breakover voltage. Additionally, these localized defects may significantly affect the spreading velocity and substantially decrease surge capabilities.
One technique used to compensate for the presence of defects and the formation of hot spots is to attempt to more uniformly distribute the avalanche phenomenon across the junction of the device. The principle of this solution is to make the switching mechanism a distributed phenomenon instead of a point phenomenon. In such a manner, when the device switches and the current flows across the junction, it is less likely to create localized hot spots at a single point of avalanching. Thus, the longevity and accuracy of the device will be increased if the avalanche charge spreads as quickly as possible across the entire junction of the device.
Thermally generated leakage current has also been known to significantly affect the breakover voltage accuracy and activation sensitivity of the devices. In addition to thermally generated or bulk leakage current effects, leakage currents across exposed surface junctions, even when protected by conventional means, have serious effects on activation sensitivity since those currents are generated in a very large magnitude. This is especially true for the large leakage currents across the blocking junction of a thyristor.
One method of distributing the breakover phenomenon across a greater portion of the junction includes the use of a buried region within a multi-layered device. The buried region directs the current through the middle of the device away from the surface, hot spots, or point defects. In this manner, a buried region decreases the leakage current on the exposed surfaces of the device, decreases the detrimental effects of hot spots and impurities within the device and increases the accuracy of the device. A single buried region, however, creates bottlenecking of the current flow through the device. This bottlenecking can substantially decrease the current carrying capacity of the device.
Another improvement in the switching characteristics of thyristor devices has been developed by the use of shorting dots in the emitter region. These shorting dots are regions in the upper base layer of the device which are contiguous to the upper metal contact. The shorting dot regions affect the sensitivity of the device and improve the accuracy of the gating characteristics as a function of temperature. Shorting dot regions improve gating characteristics by shunting out thermally generated leakage current thereby reducing the activation sensitivity of the device.
The use of a single buried region in the mid-region layer of starting silicon is disclosed in British Patent No. GB 2113907 and U.S. Pat. No. 4,967,256. The use of shorting dots in the emitter region is also disclosed in U.S. Pat. No. 4,967,256. The device disclosed in U.S. Pat. No. 4,967,256 possesses substantial bottlenecking problems and restrictions on current flow after the device is placed in a conductive state.
The use of multiple buried regions within the mid-region layer and the use of shorting dots in the emitter region is disclosed in U.S. Pat. No. 5,001,537. In U.S. Pat. No. 5,001,537, the buried region is split into a number of small areas which "should be aligned with the part of the cathode between the shorting dots." Col. 2, lines 17-19. U.S. Pat. No. 5,001,537 discloses that the principle of the invention is to break up the buried layer into an array of buried regions directly under the emitter regions (sometimes called the cathode regions). Col. 3, lines 35-38. While this arrangement may prevent some of the problems associated with bottlenecking of the current, it aligns the buried regions and the emitter regions with "dead areas" of the device. These "dead areas" often exist in the upper base layer under the emitter region or above the buried regions as a result of the higher doping required to make the buried region. This alignment of the buried region with the emitter region, therefore, causes substantial problems due to the dead areas of the device.
U.S. Pat. No. 5,001,537 discloses that the overshoot voltage for the device is "typically only 70 volts for a 90 volt device." Col. 3, line 66. This relates to a significantly high overshoot voltage value of approximately 80% over the desired breakover voltage rating. Such a high voltage overshoot could cause substantial damage to electrical equipment connected thereto by allowing voltages up to 160 V access to communication equipment. This high overshoot voltage value also restricts this device for use only as a primary protector in protection circuit configurations (typically in applications where a gas discharge tube is used).
It would therefore be advantageous to have a more sensitive and accurate solid state overvoltage protection device with lower overshoot voltage value. Further, this device should possess high surge capacity and possess a current path which substantially avoids the "dead areas" in the device. Ideally, such a device would retain a high spreading velocity and other advantages of devices currently known in the prior art.